Ethylene is produced by charging naphtha, ethane, butane or like starting material and steam into a cracking tube and heating the tube from outside to a high temperature in excess of 1000.degree. C. to crack the material within the tube with radiant heat. The material to be used for the tube must therefore be excellent in strength (especially in creep rupture strength) at high temperatures and in oxidation resistance.
The process for cracking naphtha or like material produces free carbon, which becomes deposited on the inner surface of the tube and reacts with the tube material to cause carburization and embrittle the material. Accordingly the tube material needs to have high resistance to carburization.
The cracking tube is generally fabricated in the form of a coil which comprises straight tube portions as joined to one another and to bends. Since tube components are joined together by TIG welding, MIG welding or shielded metal arc welding, excellent weldability is also required of the material.
HP improved material according to ASTM standards (0.45C-25Cr-35Ni-Nb,W,Mo-Fe) has been in wide use, for example, for making cracking tubes for producing ethylene. However, with a rise in the operating temperature in recent years, this material encounters the problem of becoming seriously impaired in oxidation resistance, creep rupture strength and carburization resistance if used at a temperature exceeding 1100.degree. C.
Accordingly, for use in operation at high temperatures of above 1100.degree. C., an alloy has been developed which comprises 0.3 to 0.8% C, 0.5 to 3% Si, up to 2% Mn, 23 to 30% Cr, 40 to 55% Ni, 0.2 to 1.8% Nb, 0.08 to 0.2% N, 0.01 to 0.5% Ti and/or 0.01 to 0.5% Zr, and the balance substantially Fe (U.S. Pat. No. 5,019,331).
This alloy is characterized in that the Cr content is held in proper balance with the content(s) of Ti and/or Zr, and that Nb, N, etc. are caused to form suitable amounts of carbonitrides to give the desired high-temperature strength.
However, we have found that the presence of at least 40% of Ni renders the alloy subsceptible to weld cracking to entail an increased likelihood of weld cracking. Nevertheless, a reduction in the Ni content results in lower carburization resistance because the oxide film formed in the vicinity of the surface of the tube and contributing to the prevention of carburization then becomes unstable, leading to lower carburization resistance. Furthermore, the reduced Ni content results in the drawback of lower strength at high temperatures.
On the other hand, investigations of creep rupture strength characteristics required of cracking tubes have revealed the following. Although the tube is actually used under high-temperature low-stress conditions (about 1100.degree. C..times.0.2-0.3 kg/mm.sup.2). the creep rupture strength has heretofore been estimated in view of the creep rupture time determined under low-temperature high-stress conditions. Thus, if a material has low creep rupture strength under low-temperature high-stress conditions, no further creep rupture test for said material was conducted as a rule under high-temperature low-stress conditions because the testing time becomes extremely longer under the high-temperature low-stress conditions, and further because it has been thought that the creep rupture strength, if high under low-temperature high-stress conditions, is correspondingly high also under high-temperature low-stress conditions.
We have found that the strength under high-stress conditions is not always in proportional relation with the strength under low-stress conditions. Thus, tubes having a high rupture strength under high-stress conditions do not always have a high rupture strength similarly under low-stress conditions.
We have further examined the relationship between the stress condition and the creep rupture time and found that the creep rupture strength characteristics are in opposite relation below and above the stress condition of about 1.0 to about 1.2 kg/mm.sup.2 when Si, Ni and Al are in a specified relation. Our research has also revealed that when having a high creep rupture strength under the condition of 1093.degree. C., 0.9 kg/mm.sup.2, cracking tubes exhibit a similarly high creep rupture strength under the actual conditions for use.
Based on the above findings, we have developed an alloy having a high creep rupture strength under high-temperature low-stress conditions and excellent resistance to carburization although reduced in Ni content.